Compositional Mapping Of The Nili Patera Feldspathic Unit: Extent And Implications For Formation

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near-infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km(2) at high confidence and an additional 41 km(2) at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near-infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light-toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify.